The present invention relates to a controller for controlling the drying of lumber in a kiln.
It is very important to dry lumber effectively and efficiently since the way lumber dries will affect its quality and final moisture content. Furthermore, the time and energy required must be considered when determining the drying method. Allowing lumber to dry naturally may not decrease the quality of the lumber substantially but will take a very long time and may not reduce the moisture content of the lumber to the desired level. If the lumber dries in a controlled environment at a constant temperature and humidity for an indefinite period of time it will eventually stabilize at a certain moisture content defined as the Equilibrium Moisture Content (EMC). The EMC is the percentage of water by weight in the lumber relative to the dry weight of the lumber. For the purpose of drying lumber, the EMC can be thought of as the amount of force applied to the lumber to draw the moisture out of the lumber, i.e. the lower the EMC, the more drying force is exerted on the lumber. By drying the lumber at varying EMC's for selected periods of time, the total drying time can be reduced, with the trade off for reducing the drying time being a possible decrease in the quality (degrading) of the lumber.
A kiln, having heating coils, steam spray nozzles, and vents for adjusting the temperature and humidity levels inside the kiln, provides such a controlled environment. A drying schedule of temperature and humidity levels regulates the drying of the lumber wherein the schedule comprises a number of steps having dry bulb temperature and wet bulb temperature set points derived from test data taken for varying EMC levels. The difference between the dry and wet bulb temperature set points, which determines the relative humidity inside the kiln, is called the wet bulb depression and indicates the drying force on the wood. Increasing the wet bulb depression increases the force on the wood.
Lumber degrades the most in the later stages of the drying schedule after reaching a point called fiber saturation which occurs when all of the water left in the lumber is confined inside the fiber structure of the wood fiber cells. Lumber is susceptible to warping, twisting, cracking, checking, staining, case hardening (collapsing of the cell walls in the outer layers of the lumber), and honeycombing (collapsing of cell walls throughout the lumber). In a kiln drying vast quantities of lumber, the lumber will dry at varying rates, and therefore it is desirable to maintain a drying force sufficient to dry the wet lumber and not substantially degrade the drier lumber. A schedule with closely controlled EMC's helps to avoid these problems.
As the lumber dries, it becomes increasingly difficult to draw the moisture out of the lumber. At a constant EMC, the lumber eventually reaches a plateau where no more moisture evaporates from the lumber, and therefore more force must be applied to the lumber by decreasing the EMC. This is accomplished by either holding the dry bulb temperature set point constant and lowering the wet bulb temperature set point, holding the wet bulb temperature set point constant and raising the dry bulb temperature set point, or a combination of changing both set points to increase the wet bulb depression.
The drying schedules for the kiln are executed by a kiln controller. One such controller involves a mechanical "cam" which rotates slowly on a shaft with the edges of the cam adjusting the dry and wet bulb temperature set points. Pneumatic information fed back from dry and wet bulb temperature sensors allows the cam to adjust the temperatures over a limited range. The mechanical cam has no capabilities for making self-correcting decisions, and a new set of cams must be cut for each schedule and may not be adjusted if the schedule isn't quite right.
An on-off controller operates similarly to a thermostat or humidity switch. If the dry and wet bulb temperatures are below the schedule values, the heating coils and spray nozzles are turned on, and if the temperatures are too high they are turned off. A vent is either opened or closed depending on the relative humidity in the kiln.
A computer driven kiln controller executes the drying schedule more effectively and with a greater degree of flexibility than either the mechanical cam or the on-off controller. The computer uses the dry and wet bulb temperature set points prescribed in the drying schedule and actual dry and wet bulb temperatures in the kiln to adjust the heating coils, steam spray nozzles, and vents to achieve the desired temperature and humidity levels. Dry and wet bulb sensors in the kiln provide the actual temperature information. Moreover, the schedule can be entered into the computer via a user interface on the controller and may be changed at any time.
For example, in a typical drying schedule the first step consists of bringing the temperature in the kiln up to an initial temperature where the drying force on the lumber is fairly mild (high EMC). The computer calculates intermediate set points which increase evenly and slowly. These set points are passed on to an algorithm that attempts to make the actual temperature agree with the set point temperatures by controlling the heating coils, steam spray nozzles, and vents. At first, the set point temperatures sent to the algorithm are higher than the actual temperatures, so the heating coils are turned on high. Shortly thereafter, a large heat rise will occur in the kiln as the coils heat the air, and the controller responds by shutting off the heating coils entirely. Eventually the air cools and the heating coils are turned on again; however, the controller has lost time in heating the kiln. The kiln temperature will continue to rise above and drop below the set points as the heating coils are turned on and off. Therefore, after the first step has elapsed, the kiln may be above or below the desired initial temperature. Achieving the initial kiln temperature in the manner described hereinabove may waste time and energy, and may not provide an accurate initial temperature.
The second step may hold at the initial conditions for a period of time estimated to be sufficient to reach fiber saturation. Then a step could ramp the dry and wet bulb temperature set points to apply a greater drying force to the lumber. The ramping step may take a few hours or a few days depending on the condition of the lumber. The last step holds at the final conditions of the ramp step for a period of time estimated to be sufficient to completely dry the lumber.
An experienced kiln operator constructs a schedule by choosing dry and wet bulb temperature set points and step lengths based on personal experience, knowledge of the lumber and the test data available for selecting the set points for desired EMC's. Still, the schedule reflects a best guess and probably won't be the optimum drying schedule. For example, if the lumber is initially very wet it may not be dry when the schedule is complete. If the lumber is very dry, a lot of time and heat energy may be wasted and the lumber may be degraded if the force on the lumber is increased after fiber saturation is reached. If the ramp step increases too fast the lumber may be damaged, and if it increases too slowly, time and energy are wasted. Therefore, an inaccurate drying schedule will degrade the lumber and increase the cost of drying.